1. Field of the Invention
This invention relates to an ultrasonic flow meter suitable for use in tube or pipe line.
2. Description of the Prior Art
As prior art techniques for measuring a flow rate of a fluid such as gases or liquids flowing in a tube or pipe line, various apparatus such as orifices and nozzles are known and of late, ultrasonic flow meters utilizing ultrasonic wave have widely been spread.
FIG. 7 shows a figure for illustrating the principle of an ultrasonic flow meters, in which when a first oscillator 102 and second oscillator 103 are fitted to a measurement pipe 101 through a constant distance L, and an ultrasonic wave is transmitted from one of the first oscillator 102 and second oscillator 103 and received by the other of the second oscillator 103 and first oscillator 102, there occurs a difference between a time t1 required when the ultrasonic wave from the first oscillator 102 reaches the second oscillator 103 and another time t2 required when the ultrasonic wave from the second oscillator 103 reaches the second oscillator 102.
When the sonic velocity in a fluid is c and a velocity of the fluid is v, the following calculation formula is concluded:
t1=L/(c+v)xe2x80x83xe2x80x83{circle around (1)}
t2=L/(cxe2x88x92v)xe2x80x83xe2x80x83{circle around (2)}
1/t1xe2x88x921/t2=2v/Lxe2x80x83xe2x80x83{circle around (3)}
v=L/2(1/t1xe2x88x921/t2)xe2x80x83xe2x80x83{circle around (4)}
That is, a formula {circle around (3)} is derived by subtracting the recipropcal number of a formula {circle around (2)} from the recipropcal number of a formula {circle around (1)} and this formula {circle around (3)} is rearranged based on v to obtain a formula {circle around (4)}. According to this formula {circle around (4)}, the flow velocity v of the fluid can be given if the distance L, time t1 and t2 are determined. When the thus obtained flow velocity v is multiplied by the inner cross-sectional area of the measurement pipe 101, the flow rate can be obtained.
As an invention based on the above described principle, for example, there is an xe2x80x9cultrasonic flow meterxe2x80x9d disclosed in JP-A-10-122923, which is constructed of such a simple structure that a measurement tube 1 is provided with ring-shaped ultrasonic oscillators 2A and 2B, and a gap with the tube 1 is filled with grease 3, 3 to acoustically combine the measurement tube 1 with the ultrasonic oscillators 2A and 2B, as shown in FIG. 3 of the same publication. These marks are same as described on this publication. The inner part of the measurement tube 1 so smooth that cleaning of contamination is not required and the apertures of the ultrasonic oscillators 2A and 2B can be rendered smaller to comply with the aperture of the measurement tube 1, thus reducing the aperture of the measurement tube 1.
FIG. 8 shows a figure for illustrating the problems of an ultrasonic flow meters of the prior art, in which when ultrasonic oscillators 2A and 2B are fitted to the measurement pipe 1, the ultrasonic oscillator 2A being used as a sending side and the ultrasonic oscillator 2B being used as a receiving side, an oscillating wave fundamentally propagates along an arrow {circle around (1)} in a fluid (medium) flowing in the measurement tube 1. During the same time, the measurement tube 1 itself functions as a propagation material and an oscillating wave shown by an arrow {circle around (2)} propagates to the ultrasonic oscillators 2B.
Thus, the oscillating wave of the arrow {circle around (2)} sometimes becomes a noise to affect unfavourably the flow measurement. When the propagation speed of the arrow {circle around (2)} is close to that of the arrow {circle around (1)}, in particular, influences thereof are larger.
It is an object of the present invention to provide an ultrasonic flow meter capable of moderating influences of the oscillating wave propagating through the measurement tube itself, whereby the above described problems of the prior art can be solved.
The above described object can be attained by an ultrasonic flow meter comprising a measurement tube for flowing a fluid to be subjected to measurement, said tube being constructed of a metallic or metal-like material capable of propagating ultrasonic wave and being free from obstacles hindering the flow of the fluid in the tube, a first oscillator fitted to the outer circumference of the measurement tube, a second oscillator fitted to the outer circumference of the measurement tube at a predetermined interval along the flow of the fluid from the first oscillator and an acoustic filter fitted to the measurement tube for cutting off high frequency range of the oscillating wave propagating through the oscillating tube, in which the flow rate of a fluid is measured based on a time difference between a time required when the ultrasonic wave from the upstream oscillator reaches the downstream oscillator and another time required when the ultrasonic wave from the downstream oscillator reaches the upstream oscillator.